LETTERS
PUBLISHED ONLINE: 25 JANUARY 2009 DOI: 10.1038/NGEO417
Timing of crystallization of the lunar magma
ocean constrained by the oldest zircon
A. Nemchin1 *, N. Timms1 , R. Pidgeon1 , T. Geisler2 , S. Reddy1 and C. Meyer3
The Moon is thought to have formed through the consolidation
of debris from the collision of a Mars-sized body with the Earth
more than 4,500 million years ago. The primitive Moon was
covered with a thick layer of melt known as the lunar magma
ocean1 , the crystallization of which resulted in the Moon’s
surface as it is observed today. There is considerable debate,
however, over the precise timing and duration of the process
of magma ocean crystallization. Here we date a zircon from
lunar breccias to an age of 4,417 ± 6 million years. This date
provides a precise younger age limit for the solidification of
the lunar magma ocean. We propose a model that suggests
an exponential rate of lunar crystallization, based on a
combination of this oldest known lunar zircon and the age of the
Moon-forming giant impact. We conclude that the formation
of the Moon’s anorthositic crust followed the solidification of
80–85% of the original melt, within about 100 million years of
the collision. The existence of younger zircons2 is indicative of
the continued solidification of a small percentage of melt for an
extra 200–400 million years.
Fractional crystallization of the lunar magma ocean (LMO)
involved the early density-driven separation of mafic cumulates
and flotation of a plagioclase-rich lunar crust represented by
ferroan anorthosite1 . Subsequent crystallization of ilmenite from
the remaining portion of the LMO (ref. 1) left a residual liquid
enriched in highly incompatible elements. This liquid formed the
enriched reservoir referred to as KREEP (from high concentrations
of potassium, rare-earth elements and phosphorus3 ).
A precise determination of the timing of fractional crystallization
of the LMO has been inhibited by the susceptibility of Sm–Nd
and other systems to the partial resetting during the later thermal
pulses associated with the meteorite impacts. As a result, the Sm–Nd
mineral isochrons constrained for the ferroan anorthosite samples
show a wide spread of ages between 4,560 ± 70 Myr (ref. 4) and
4,290 ± 60 Myr (ref. 5). The best estimate for the age of ferroan
anorthosites determined as 4,456 ± 40 Myr from the combination
of mafic minerals in all analysed samples but excluding plagioclase
data that are partially disturbed6 has another inherited problem as
it assumes that all samples have been formed at the same time.
Another way that has been used to constrain the timing of
the LMO differentiation is using model ages of rocks derived
from different reservoirs in the lunar mantle. In particular, a
KREEP-rich source is recognized as an essential part of late-stage
crystallization of the LMO, and model ages of KREEP formation
have been estimated as ∼4,600 Myr by Rb–Sr analysis of lunar
soils7 , ∼4,420 Myr from U–Pb systematics of highlands rocks and
a basalt sample8 and ∼4,360 Myr from the Sm–Nd model ages of
KREEP samples9 . An average of the model age for KREEP was
estimated as 4,420 ± 70 Myr (1σ uncertainty)10 . Recent tungsten
isotope data on metals from low- and high-titanium mare basalts as
well as two KREEP-rich samples11 suggest that the last equilibration
of the LMO, which is possible only up to a critical point when
about 60% of the melt is solidified, occurred after 4,507 Myr ago
(60 Myr after the formation of the Solar System). This result is in
agreement with the 146 Sm–142 Nd model age of the LMO (ref. 11),
which is based on the combined 147 Sm–143 Nd and 146 Sm–142 Nd
systems in lunar basalts and implies a 238+56
−40 Myr (ref. 12) to
215+23
−21 Myr (ref. 13) time interval for lunar mantle formation.
Despite the general agreement between the model ages determined
using different isotope systems, their accuracy is limited by the
models and the timing of LMO crystallization remains loosely
constrained to the first 250 Myr of lunar history.
Both isotopic resetting and model dependence problems
associated with numerous previous attempts to place limits on the
time of LMO crystallization can be avoided by using the U–Pb
system in zircon2,14 , which is well known for its stability under a
variety of extreme conditions. Growth of zircon in melts is governed
by zircon saturation, which can be achieved only in a mafic magma
initially enriched in zirconium15 . Consequently, the presence of
zircon in the lunar samples is linked to the initial enrichment
of the magma in the KREEP component (that is, KREEP must
form on the Moon before zircon can appear in any rock type).
Therefore, the oldest zircon defines a younger limit for the time
of KREEP formation.
Here we report the oldest zircon crystal found on the Moon so
far, which is located in the matrix of Apollo 17 clast-rich impact melt
breccia 72215, in the thin section 72215, 195. The 0.5 mm grain lacks
well-developed crystal faces and contains several brittle fractures
(Fig. 1), and we thus consider it to be a relict fragment of a larger
grain that was incorporated into the host breccia.
Forty-one secondary-ion mass spectrometry U–Pb analyses were
made on this grain (see Supplementary Information, Table S1,
Fig. 2a). The results indicate a complex pattern of isotope resetting
that systematically varies with the microstructural features of the
grain (see Supplementary Information, Table S1; Fig. 1). These
microstructural features are a combination of primary magmatic
characteristics and a different degree of self-irradiation damage
highlighted by the variable birefringence and cathodoluminescence
emission, as well as deformation patterns revealed by crystallographic orientation analysis of electron backscatter diffraction
(EBSD) patterns. The observed overall decrease in 207 Pb/206 Pb ages
correlated with an increase of the local misorientation determined
for each sensitive high-resolution ion microprobe (SHRIMP) spot16
(Fig. 2b), indicates that this differential resetting of the U–Pb
system occurred as a result of impact-related plastic deformation,
1 Department
of Applied Geology, Western Australian School of Mines, Curtin University of Technology, Bentley, Western Australia, 6102, Australia,
für Mineralogie, Westfälische Wilhelms-Universität, Corrensstr 24, 48149, Münster, Germany, 3 NASA Johnson Space Center, Houston, Texas,
77058, USA. *e-mail: [email protected].
2 Institut
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133
LETTERS
NATURE GEOSCIENCE DOI: 10.1038/NGEO417
a
b
30
29
14
15
13
27
28
21
7
26
19
23
22
32
31
33
2
9
i
24
25
1
16
10
8
4
41
39
37
20
6
12
Age (Gyr) 38
<4.35
11
4.350
4.375
4.400
>4.40
i
200 µm
c
36
35
5
40
3
34
17
18
200 µm
d
12°
100
200 µm
10
0
200 µm
Figure 1 | Microstructure of the zircon grain from lunar breccia 72215, 195. a, Cross-polarized-light photomicrograph showing sector zones and faint
compositional growth zones (inset). b, Panchromatic cathodoluminescence image with superimposed mean U–Pb ages for individual SHRIMP analyses.
c, Map showing variations in EBSD pattern quality (band contrast) from poor (black) to good (white). d, Map derived from EBSD data showing variations in
crystallographic orientation relative to the mean reference orientation (red cross).
an interpretation that is consistent with trace-element variations
recorded in other deformed zircons16,17 .
All 41 U–Pb analyses are distributed along concordia between
4,418 ± 8 and 4,331 ± 16 Myr (uncertainties are 2σ ; Fig. 2a).
The four oldest analyses, from undeformed parts of the grain,
form a coherent group on a concordia plot (Fig. 2a) with a
concordia intercept at 4,420 ± 15 Myr and an average 207 Pb/206 Pb
age of 4,417 ± 6 Myr. We interpret this age as the age of
zircon crystallization. The five youngest analyses form a coherent
group in the 207 Pb/206 Pb versus 238 U/206 Pb diagram (Fig. 2a),
defining a concordia intercept age of 4,334 ± 10 Myr for the
common-lead uncorrected data and an average 207 Pb/206 Pb age
of 4,333 ± 7 Myr for the Stacey–Kramers modern-lead corrected
data. These analyses correspond to areas of moderate luminescence,
good EBSD pattern quality and low uranium and thorium
concentrations (Fig. 1). Importantly, these analyses are also from
areas where the deformation bands intersect and/or have high
misorientation, suggesting deformation-related lead loss. It is
evident that the most deformed areas of the grain have suffered
the greatest lead loss, and we interpret the concordia intersection
age as a reflection of mobility of the U–Pb system in the grain
during an impact, although the resetting can be incomplete.
The remaining intermediate ages are from areas of moderately
134
strained parts of the grain, and probably reflect a partial resetting
of the U–Pb system.
Our results indicate that the KREEP source formed by
4,417 ± 6 Myr and it follows that crystallization of the LMO
was almost completed by this time. The zircon age is almost
100 Myr older than the age calculated from combined 142 Nd–143 Nd
systematics of lunar basalts and highland rocks12,13 . These later
estimates, however, are based on the assumption that the separate
mantle reservoirs have been formed at the same time and had
similar initial isotopic compositions of neodymium. This may
not be the case, even for KREEP magmas and the source of
high-titanium basalts. Both formed last in the LMO crystallization
sequence and largely define the slope of the combined 142 Nd–143 Nd
isochrons. Nevertheless, the formation of the KREEP source at
4,417 ± 6 Myr is in agreement with the age of 4,456 ± 40 Myr
determined for the ferroan anorthosite samples6 , even though the
ages are not completely resolved within the errors.
A combination of the KREEP minimum formation age of
4,417 ± 6 Myr and other data reflecting different stages of LMO
evolution enables us to model the history of magma ocean differentiation and crystallization on the Moon, and two endmember
models are presented (Fig. 3). Both models are constrained by
the new 4,417 ± 6 Myr zircon age, defining a minimum age for
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LETTERS
NATURE GEOSCIENCE DOI: 10.1038/NGEO417
a
Intercept: 4,420 ± 15 Myr
MSWD = 0.11; probability = 0.90
4,460
207Pb/206Pb age = 4,417 ± 6 Myr
MSWD = 0.08; probability = 0.97
207Pb/206Pb
0.57
4420
0.55
4380
0.53
4,300
1.02
1.06
40
20
Oldest
limit for
plagioclase
appearence
Ilmenite cumulate
KREEP
4,420
4,380
4,340
10
20
30
4,400
Age (Myr)
4,300
4,200
Figure 3 | LMO crystallization paths based on the available chronological
data. Solid line projected through the points representing (1) initial
formation (100% melt—182 W age; ref. 11), (2) mean time of lunar crust
formation (30% melt—143 Nd age; ref. 6) (3) KREEP formation (5–7%
melt—age from this study), (4) time of cessation of magmatic activity in
the Serenitatis region (2.5–3.5% melt—age estimate from zircon
distribution patterns2 ); dotted line based on (1) and (2) and the assumption
of a turbulent convection in the LMO resulting in the fast initial cooling; the
yellow circle represents predicted formation of the lunar crust compatible
with such fast cooling of the LMO. Data-point error crosses are 2σ .
b
0
Oldest limit for
anorthosite crust formation
4,500
1.10
238U/206Pb
age (Myr)
60
0
207Pb/206Pb
0.51age = 4,333 ± 7 Myr
0.90probability
0.94 = 0.990.98
MSWD = 0.05;
207Pb/206Pb
Youngest limit for
olivine¬pyroxene cumulates
80
4340
Intercept: 4,334 ± 10 Myr
MSWD = 0.08; probability = 0.97
4,300
100
Melt remaining (%)
0.59
40
50
Local misorientation (°) /alpha dose (×1010)
Figure 2 | U–Pb SHRIMP data for the zircon from the breccia thin section
72215, 195. a, Tera–Wasserburg concordia diagram. Data are not corrected
for the initial lead. Blue ellipses represent the four oldest analyses; red
ellipses represent the five youngest analyses; yellow ellipses represent
analyses with intermediate U–Pb ages. MSWD: mean square weighted
deviation. Data-point error ellipses are 2σ . b, Age versus ‘local
misorientation’ value determined at each SHRIMP spot from EBSD map
data by calculating the mean misorientation between a central point and its
nearest neighbours on an 11 × 11 pixel grid (13.2 µm × 13.2 µm area; ref. 16).
Local misorientation data were normalized to alpha dose to account for the
radiation damage. The resultant local misorientation values are interpreted
to reflect lattice distortions associated with crystal-plastic deformation.
Error bars are 2σ .
formation of lunar KREEP at a late stage in the crystallization
of the LMO. Both are also based on the assumption that the
LMO formed as a result of fast accretion following the giant
impact1 and, therefore, the age of LMO formation is similar to
the age of the Moon. The best current estimate of the age of
the giant impact based on the Hf–W data is 62+90
−10 Myr after the
formation of the Solar System11 . These data place an older limit
of LMO formation of 50 Myr after the first condensation in the
solar nebula (4,517 Myr ago). A simple model of LMO evolution
(Fig. 3, solid line) suggests a sequential fractionation of olivine →
orthopyroxene±olivine → olivine+clinopyroxene±plagioclase →
clinopyroxene + plagioclase → clinopyroxene + plagioclase
+ ilmenite assemblages. However, the assumption of sequential
fractionation of mineral phases throughout the whole LMO is
probably an oversimplification because it is likely that: (1) a significant temperature difference would exist between the lower and
upper parts of the LMO, (2) the appearance of different minerals
on the liquidus is unlikely to be contemporaneous in different
parts of the magma ocean, (3) convection can prevent effective
removal of minerals from the liquid and (4) the formation of
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an insulation lid can change the cooling regime of the LMO. A
more complex model of LMO crystallization (Fig. 3, dotted line)
involves rapid initial cooling of the magma ocean as a result of
vigorous turbulent convection18 , which results in solidification of
a substantial proportion of LMO without significant fractionation.
This was followed by fractionation limited to the relatively thin top
layer of the LMO owing to much slower cooling resulting from a less
vigorous convection regime, and possibly formation of a thermally
insulating surface lid.
Nevertheless, both models combined with the available
chronological data suggest that ilmenite-bearing cumulates precipitated after about 90% of LMO crystallization, leaving a few per cent
of residual KREEP melt by 4,417 ± 6 Myr ago. These data suggest
that the main volume of the LMO solidified within about 100 Myr.
The age distribution patterns obtained for numerous zircon grains
from Apollo 17 and 14 breccias2 suggest that the residual small
volume fraction of the LMO liquid could have cooled slowly over
the subsequent 400–500 Myr, probably sustained by the internal
heating related to radioactive decay. These patterns indicate gradual
shrinking of a semi-molten KREEP reservoir towards the centre
of Procellarum KREEP terrane2 , and that by about 4,250 Myr ago
the KREEP reservoir solidified under the area occupied by the
Serenitatis basin, but continued to be active closer to the middle
of Procellarum KREEP terrane near the Imbrium basin until about
3,900 Myr ago. Assuming that the thickness of the KREEP source
is approximately constant throughout the Procellarum terrane, this
accounts for an extra reduction in the residual proportion of KREEP
melt of about 50% by 4,250 Myr ago.
Despite the precise fixation of the timing of the last stage of LMO
crystallization by our results, the timing of plagioclase appearance
in the crystallization sequence remains imprecise. Estimates for
the appearance of plagioclase on the liquidus vary from about
60% to 80% of LMO crystallization, depending on the assumed
bulk Al content of the LMO (refs 19,20). Assuming sequential
crystallization of minerals (Fig. 3, solid line) and using available
geochronological data for the ferroan anorthosite samples, 70%
of crystallization of the LMO is necessary before plagioclase can
become a liquidus phase. In the more complex model (Fig. 3, dotted
© 2009 Macmillan Publishers Limited. All rights reserved.
135
LETTERS
NATURE GEOSCIENCE DOI: 10.1038/NGEO417
line), the lunar crust formed after crystallization of 80–85% of
the LMO. However, both estimates are within the uncertainties
associated with the relatively imprecise estimate of the age of the
ferroan anorthosites. The large uncertainty of this age also results in
the large range (anywhere between 20 and 100 Myr) for the possible
duration of plagioclase flotation. As a result, further refinement of
the models awaits more precise determination of the age of lunar
anorthosite formation.
Methods
The sample is a polished thin section of breccia 72215 prepared by NASA
(National Aeronautics and Space Administration). The microstructure of the
zircon was characterized by cathodoluminescence imaging and EBSD mapping
using a scanning electron microscope at Curtin University of Technology, Perth,
Western Australia. Collection of EBSD data was processed using the procedures
developed for zircon21 . Slip systems were resolved from crystallographic orientation
data using simple geometric models of low-angle boundaries17 .
U–Pb data were obtained using a SHRIMP at the John de Laeter Centre
of Mass Spectrometry, Curtin University of Technology following the standard
analytical procedure described elsewhere14 . Pb–U ratios were normalized to the
564-Myr-old Sri Lankan zircon CZ3 analysed in a separate mount. Common lead
was corrected for using modern Stacey and Kramers lead, following the conclusion
that a substantial proportion of common lead in the lunar thin sections results from
the surface contamination2 . Regardless, of the selection of common lead for the
correction, a very low proportion of 204 Pb in the thin section 72215, 195 makes the
calculated ages insensitive to the uncertainty in the common lead.
Received 10 November 2008; accepted 19 December 2008;
published online 25 January 2009
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Acknowledgements
In particular we would like to thank the astronauts of Apollo 17 for collecting the sample.
The project was supported by the office of R&D department at Curtin University of
Technology. Imaging was supported by the Australian Research Council Discovery Grant
DP0664078 to S.R. and N.T.
Additional information
Supplementary Information accompanies this paper on www.nature.com/naturegeoscience.
Reprints and permissions information is available online at http://npg.nature.com/
reprintsandpermissions. Correspondence and requests for materials should be
addressed to A.N.
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